Correlated studies of the same chromosome in life and in the electron microscope are proposed. The first aim is progress toward understanding chromosome distribution. We would examine the initial interactions of chromosomes with the spindle, seeking information on the origin of kinetochore microtubules and their integration with those of the rest of the spindle. Second, we would investigate spindle structure after chromosome micromanipulation. The aim is to create situations in which the microtubules associated with a single chromosome are readily identifiable and in which mechanical linkage between microtubules is directly revealed. Quantitative studies of mitotic forces and spindle mechanical properties are also proposed. We would use glass fibers calibrated to produce a known force when the tip is deflected by a given extent. We would attempt to determine the maximum force produced by the spindle, the resistance of microtubules to bending, and the relationship between chromosome velocity and the resistance to movement.